Set ammo type and whether or not it uses magazines or individual shots on the weapon details sheet. When you click the reload button (next to the ammo counter on your character sheets combat tab), it will refill your weapon with 1 magazine or as many shots as fit or are left in your in inventory.

Ammo must be carried or equipped to be considered for reloading.

Thanks to Mike Serfass for updating the extension with a nicer layout and locking the Ammo related fields when a weapon is locked.

For more information see the forum thread.